in the sector shaft meshes with the helical track on the cam, and the two together are responsible for providing the variable ratio. No external lockstops are provided as they are incorporated into the steering box. Control valve In common with other power steering systems, the control valve is of the rotary type and consists of a valve rotor and torsion bar. The valve rotor is the input shaft to the steering gear and has six longitudinal grooves machined into its outer surface. When no load is applied to the steering wheel, these grooves lie between six grooves in the valve sleeve and no hydraulic assistance is applied. When steering effort is applied at the wheel it is transmitted to the rotor, which transmits the effort to the hour glass cam by means of the torsion bar. The rotor is normally centred in its sleeve by the torsion bar but manual steering effort twists the torsion bar, permitting the rotor to turn within its sleeve, opening the way for hydraulic fluid pressure to be applied to one or other side of the servo piston, thus assisting in turning the front wheels. Pressure pump The hydraulic pump is of the vaned rotor type, typically of Saginaw manufacture, and is connected to the steering box via flexible feed and return hoses. The fluid reservoir is incorporated in the pump's end cover. Typical system operating pressures are in the range 7.8 - 8.8 MPa, the operating medium being automatic transmission fluid. Operation The varying pitch of the cam results in the variable steering ratio, with the lowest gearing being at the straight ahead, rising rapidly to either lock. The rise in gearing (which in the Jaguar 420 was equivalent to a drop in ratio from 21.6:1 to 13:1) occurs almost entirely within the first half turn of the steering wheel from the straight ahead position. The effect is to give very light and relaxed steering at the straight ahead, with increased reaction when cornering. Benefits and drawbacks The major benefit of the Variomatic system is the reduction in the amount of "arm twirling" and effort required when steering, particularly when a lot of cornering is involved. For example, the Variomatic system fitted as an option to the Jaguar 420 required only 2-7/8 turns of the steering wheel from lock to lock, which compares favourably with 4-1/4 turns between locks for the manual Burman